It's great to see so many people in the comments completely ignore that stories need conflict and
that if everything can be resolved in a single chapter, nobody would read such a boring story.
I didn't know having an 'adult' mind was a superpower and can fix everything with a snap of the fingers!
I mean it's not like the MC is adopted and not accepted by the family, or the nobility.
Or that the aunt is also an adult.
Or the MC is a little girl and has no power in any way, shape or form.
Or that even her brothers don't particularly love her.
Or that she hates this family and wants out.
As soon as the MC puts a toe out of line she'll be immediately beaten down or worse, thrown back into the streets because she's NOT part of the family.
MC is an outsider and if she wants to survive, she has to keep her head down until she gets the backing of someone more powerful.
They broke Eugene's leg.
They broke the DUKE'S LEG.
They might have our MC have a fatal 'accident' if she actually does try to get rid of the aunt impulsively.
Slow and steady wins the war.